Electrode for an electrical discharge device



"Dec. 16, 1969 J. A. c. JANSEN ET AL ELECTRODE FOR AN ELECTRICAL DISCHARGE DEVICE Filed March '7, 1967 INVENTORS JOHANNES A.C.JANSEN TJ EPKE H. E KKELBOO M w El AGENT United States Patent Int. Cl. Htllj 17/20, 61/12 US. Cl. 313-184 3 Claims ABSTRACT OF THE DISCLUSURE An electrode hermetically sealed in the wall of a highpressure electrical discharge lamp having two portions joined in the wall of the lamp by a material having a higher melting point than that of the Wall material.

The invention relates to an electrode which is hermetically sealed in the wall of an electrical discharge device, e.g., a high-pressure lamp.

The electrode end of such lamps facing the discharge usually consists of thoriated tungsten (i.e., tungsten containing a few percent of thorium). However, when thoriated tungsten is sealed into a lamp wall of high-meltingpoint glass, for example, quartz glass, thorium reaches the surface due to the required high sealing temperature,

which may give rise to difliculties with respect to a vacuum-tight closure. Therefore, the electrode is made of two parts, the part passing through the lamp Wall consisting of pure tungsten.

The said electrode parts should be joined in an electrically and mechanically reliable manner. If the soldering material used for this purpose consists of a metal, for example, platinum, the melting temperature of which is approximately equal to or lower than the softening temperature of the glass, during operation of the lamp at temperatures which may lie just below the softening point of the glass, this metal may arrive in the discharge space due to evaporation, which is undesirable.

In contradistinction to molybdenum (melting temperature 2600 C.) and molybdenum alloys, other metals of very high melting point such as tantalum, niobium, etc., proved to be unsuitable for use as Soldering material, since they absorb hydrogen (developed from the glass or during the process of exhausting the lamp), which results in a brittle structure of the soldering area detrimental to the mechanical strength of the electrode.

It is an object of the invention to provide an improved electrode for a high-pressure electrical discharge lamp.

Further objects of the invention will appear as the specification progresses.

In accordance with the invention an electrode for a high-pressure gas and/or vapor discharge lamp consists of two electrically series-connected parts. The part which passes through the Wall of the lamp consists of thoriated tungsten, while the two parts of the electrode are joined to each other solely by means of an intermediate layer of molybdenum or of a molybdenum alloy the melting point of which lies considerably higher than the softening temperature of the high-melting-point glass.

The invention will now be described more fully with reference to the drawing, which shows an embodiment of the invention.

In the drawing, a rod-shaped electrode of a short are Xenon discharge lamp of 2 to 3 kw. (at a lamp current of 100 to 150 a.) faces the discharge, has a conical free end and consists of thoriated tungsten; in operation, the

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pressure in this lamp is approximately 25 atmospheres and in the cold state approximately 8 to 10 atmospheres.

The electrode part 1 is joined through a molybdenum layer 2 to a similar rod-shaped part 3 of pure tungsten which in the case shown has the same diameter of approximately 4.5 mms. as the part 1 and is in line With the latter. The joint between the two parts 1 and 3 is established in that the two parts with an interposed molybdenum plate are heated at high temperature, for example, by means of a flame or high-frequency energy.

Part 3 is joined in a vacuum-tight manner through intermediate glass parts 4, 5 and 6 to the lower portion 7 of the quartz glass bulb 8.

In the case shown, the soldering area 2 is located in a narrowed cylindrical portion 9 of the lamp so that the soldering area 2 of the electrode 1-3 is protected from excessively high bending loads resulting, for example, from shocks during the transport of the lamps.

The soldering area 2 is preferably located at a distance from the conical end of the electrode part 1 such that the discharge cannot terminate at this area. In the present case, the distance between the conical end and the soldering area is approximately 40 mms., while this soldering area is located at a distance of approximately 15 mms., from the upper end of the cylindrical portion 9.

While the invention has been described in connection with specific examples and applications thereof, other modifications will be apparent to those skilled in the art without departing from the spirit and scope of the invention which is defined in the appended claims.

What is claimed is:

1. In a high-pressure electrical discharge lamp a rodshaped electrode which is passed in a gas-tight manner through the wall of the lamp consisting of a high-meltingpoint glass, said electrode consisting of two electrically series-connected parts, one part, passing through the wall of the lamp, consisting of pure tungsten and the other part, joined thereto and located inside the lamp, consisting of thoriated tungsten, the two parts of the electrode being joined together solely by means of an intermediate metal disc containing molybdenum, the melting point of said metal disc being substantially higher than the softening temperature of the glass.

2. A lamp as defined in claim 1, in which the wall consists of quartz glass.

3. A lamp as defined in claim 1, in which said metal disc consists of molybdenum.

References Cited UNITED STATES PATENTS 2,341,716 2/1944 Herdman 313-332 X 2,400,082 5/1946 Eitel et al 313-332 X 2,567,491 9/1951 Mitchell 313-184 2,716,584 8/1955 Retzer 316-19 3,014,156 12/1961 Osterhammel et a1. 313-184X 3,132,279 5/1964 Lewin 313-218 X 3,390,296 6/1968 Smith 313-253 X OTHER REFERENCES Materials and Techniques for Electron Tubes, by Walter H. Kohl, 1962, Reinhold Publishing Corp., New York, N.Y., section 7 entitled Tungsten and Molybdenum Joints, appearing on pp. 386, 387, 388 relied upon.

JAMES W. LAWRENCE, Primary Examiner PALMER C. DEMEO, Assistant Examiner US. Cl. X.R. 

